In the context of motor control applications, or applications relating to the control of any electric machine or generally to the control of a load, regardless of whether it is inductive or resistive, it is of course essential to be able to determine whether the load is present or not.
Specifically, in particular in the automotive field but also in other technical fields in which a resistive or inductive load is driven using an electronic circuit in which, ultimately, a control transistor transmits a control signal to said load, the ability to detect any occurrence of an open-load situation is essential. The ability to make this diagnosis quickly and reliably is also required.
Thus, to detect faults in a circuit, such as the breakage of a wire or the disconnection of the load, open-load (according to the term often used by those skilled in the art) detection functions have been developed.
In practice, according to the prior art, there are two circumstances under which an open load is sought:                the most common, an open load is sought “ON load”. In this case, the load is being controlled, and it is sought to start up the load, i.e. to pass a current through the branch formed by the control transistor and the load;        it is also possible to search for an open load “OFF load”: in this case, it is also sought to pass a current through the load, but it must be a very “small” current that will not trigger the operation of the load (which may be complicated: in particular, the current to be circulated must preferably be at least higher than contact deoxidation current) and this also requires the presence of an additional circuit, comprising an external current source for managing said small current.        
In both cases, if a sufficient current effectively flows through the load, and hence through the branch formed by the control transistor and the load, a diagnosis of a load being present may be established. In the opposite case, it is possible that there is an open-load situation.
In the context of the present invention, the proposed method is implemented “ON load”.
In this case, according to the prior art, several methods for detecting a potential open load are known, the method used depending on the type of load: resistive or inductive.
In the case of a resistive load, since the control transistor is closed, it is sought to pass a current through the load. If a current is measured between the output of the control transistor and the load, and if the intensity of this current is higher than a predetermined positive threshold, then a diagnosis that there is no open load may be established. If no current having an intensity higher than the predetermined positive threshold is detected, then a diagnosis that there is an open-load, situation may potentially be established.
In the case of an inductive load, a potential control current takes time to be established on the branch formed by the control transistor and the load. Because of this, it is necessary to wait a certain amount of time before being able to establish a diagnosis relating to a potential open-load situation. In practice, when the control transistor is closed in the presence of the load, the load is discharged into the electronic circuit. If the control transistor is opened, the load is discharged into a parasitic or flyback diode. Consequently, if a reversal of the direction of the current between the control transistor and the load is detected when said control transistor is closed, then a diagnosis that the load is present may be established. Conversely, if the load is not present (open load), then the load is not discharged into a diode when the control transistor is opened and no current reversal can be detected, allowing a diagnosis of an open load to be established.
The first method, consisting in seeking to measure a current having an intensity higher than a predetermined positive threshold, is quite effective in the case of a resistive load. However, in the case of an inductive load, the time required to establish a confident diagnosis is detrimental, since it is generally necessary to know immediately, or as quickly as possible, whether the load is open or not when load control is implemented.
In the prior art, this paradox results in numerous false detections: an open load may thus be diagnosed when in fact the load is present, the time needed to establish a current having an intensity higher than the predetermined positive threshold proving to be too long for load control.
Additionally, in the time taken for the current to be established in the branch formed by the control transistor and an inductive load, it is possible that load control will have reversed the direction of the control current, such that the time needed before a current having an intensity higher than the predetermined positive threshold is detected may be even longer.
To overcome this drawback, methods have been developed and belong to the recent prior art. To limit the time needed to establish a diagnosis of an open load, these techniques make provision for influencing load control to force it to control the load so that a faster diagnosis may be established. Otherwise, it is often necessary to wait, potentially for a long time from the point of view of the application, for load control to generate commands allowing conditions making it possible to establish the diagnosis of an open load to be present.
The major drawbacks of this known principle, consisting in seeking to measure a current having an intensity higher than a predetermined positive threshold so as to establish a diagnosis of an open load, therefore reside in the substantial amount of time needed to obtain a reliable diagnosis and/or in the need to influence the commands that are normally generated by load control to more rapidly obtain the conditions allowing such a diagnosis to be established.
The second family of known techniques consisting, in the case of an inductive load, in detecting a reversal of the current on the branch formed by the control transistor and the load when closing/opening said control transistor is more complex to implement and furthermore still involves a potentially long time before it is capable of establishing a reliable diagnosis. Specifically, it is necessary for the load to be properly charged for it to be possible to detect a discharge thereof, and hence the reversal of the current, when the control transistor is closed after it having been open.
Otherwise, it is not possible to detect the load. In this case too, it is therefore necessary, before establishing a reliable diagnosis, for the control transistor to have been closed long enough to allow the load to charge.
The known solutions, as in the preceding case, consist in waiting for the required time or in influencing load control.